In recent years, requests for improving data rate, capacity, diversification and quality for mobile communications are strengthening more and more. Especially, from the viewpoint of achieving high speed and large capacity, the communication technology of the MIMO scheme receives attention. In the MIMO scheme, each of a plurality of transmission streams propagates in a space in different ways, so as to improve transmission speed or signal quality. The receiving side needs to separate the plurality of streams properly. Several techniques have been proposed as the signal separation method. As examples, there are Minimum Mean Square Error (MMSE) method, Maximum Likelihood Detection (MLD) method, computation amount reducing type MLD (example: QRM-MLD) method, and the like.
In view of improving throughput of the whole system by performing transmission using a proper transmission rate according to channel states that change over time, the Adaptive Modulation and Channel Coding (AMC) scheme may be performed.
FIG. 1 is a diagram for explaining the principle of the AMC scheme. FIG. 1 schematically shows throughput that can be achieved by specific MCS from the viewpoint of signal quality SINR. The achievable transmission rate increases in an ascending order of MCS1, MCS2 and MCS3. MCS is an abbreviation of Modulation and Coding Scheme. The data modulation scheme may be determined like M1=QPSK, M2=16 QAM, M3=64 QAM, . . . , for example. The channel coding rate may be determined as R1=1/8, R2=2/8, R3=3/8, . . . and the like. Combinations of the data modulation scheme and the channel coding scheme are predetermined according to achievable transmission rates (MCS1, MCS2, . . . , for example). Quality of a channel state can be estimated by a degree of signal quality such as SNR. In general, the better the signal quality is, the higher the transmission rate that can be used becomes, so that throughput increases. Conversely, when the signal quality is bad, only a low transmission rate can be used, so that the throughput becomes small. In the case of the example shown in the figure, although either of MCS1 and MCS2 can be used for signal quality q1, MCS2 (the data modulation scheme is 16 QAM, and the channel coding rate is 1/2) should be used from the viewpoint of achieving a higher transmission rate (throughput). The determination criterion for selecting MCS may be, for example, a criterion for increasing throughput of individual users or a criterion for increasing throughput of the whole system. Or, conversely, MCS1 may be selected in favor of certainty of data transmission. Accordingly, since a transmission rate suitable for the channel state is properly used according to the channel state in AMC, to select proper MCS is also referred to as link adaptation.
In the MIMO scheme, there are a plurality of transmission streams, and each of them is transmitted with each different channel state (that is, propagation route). Therefore, in the case of the MIMO scheme, there is a room for performing AMC for each transmission stream.
FIG. 2 shows an example of a system in a case where two antennas are provided for each of transmission and reception, and the MLD method is used for signal separation. In the example shown in the figure, the stream #1 is channel-coded, interleaved, and data-modulated, and after that, the stream #1 is transmitted from the antenna #1. Similarly, the stream #2 is also channel-coded separately, interleaved, data-modulated, and after that, the stream #2 is transmitted from the antenna #2 separately. In the receiving side, processing of signal separation is performed on the signals received by the antennas #1 and #2 so that the signals are separated into each stream. Each separated stream is deinterleaved and channel-decoded. On the other hand, channel estimation is performed based on the received signal before channel separation. As a result, link adaptation is performed. The decision result (proper MCS) of the link adaptation is fed back to the transmission side, so that the result is used for transmission of streams after that.
In the case of the system example shown in FIG. 2, the received signal [r1 r2]T before signal separation can be represented as the following equation (wherein “T” indicates transposition).
                              r          ≡                      [                                                                                r                    1                                                                                                                    r                    2                                                                        ]                          =                                            [                                                                                          h                      11                                                                                                  h                      12                                                                                                                                  h                      21                                                                                                  h                      22                                                                                  ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                  ]                                +                      [                                                                                n                    1                                                                                                                    n                    2                                                                        ]                                              (        1        )                                                          ⁢                  =                                                    h                1                            ⁢                              s                1                                      +                                          h                2                            ⁢                              s                2                                      +            n                                              (        2        )                                                          ⁢                              =                          Hs              +              n                                ⁢                                          ⁢                                    E              ⁡                              [                                  nn                  H                                ]                                      =                                          σ                2                            ⁢              I                                                          (        3        )            
Meaning of each symbol is as follows.    ri: a signal received by an i-th receiving antenna    hij: channel variation between j-th transmission antenna and i-th receiving antenna (channel matrix element)    ni: noise at the i-th receiving antenna    sj: symbol of j-th stream, E{|sj|2}=1 (E represents expected value)    σ2: noise power
When the MMSE method, instead of the MLD method, is used for signal separation, signal quality SINR for each of streams after signal separation can be derived easily. More particularly, the signal quality SINRMMSE(1)for the stream #1 and the signal quality SINRMMSE(2)for the stream #2 can be calculated as shown in the following equations.
                                          SINR            MMSE                    ⁡                      (            1            )                          =                                            h              1              H                        ⁢                          R                              -                1                                      ⁢                          h              1                                            1            -                                          h                1                H                            ⁢                              R                                  -                  1                                            ⁢                              h                1                                                                        (        4        )                                                      SINR            MMSE                    ⁡                      (            2            )                          =                                            h              2              H                        ⁢                          R                              -                1                                      ⁢                          h              2                                            1            -                                          h                2                H                            ⁢                              R                                  -                  1                                            ⁢                              h                2                                                                        (        5        )                                R        =                  (                                                    h                1                            ⁢                              h                1                H                                      +                                          h                2                            ⁢                              h                2                H                                      +                                          σ                2                            ⁢              I                                )                                    (        6        )            
Therefore, based on the principle shown in FIG. 1, MCS suitable for each stream can be easily determined for each stream. As to the stream #1, a proper MCS can be determined using SINRMMSE(1), and, as to the stream #2, a proper MCS can be determined using SINRMMSE(2).
On the other hand, as to the MLD method, all combinations of symbol groups and MCS are searched, so that an optimum symbol group and MCS are estimated. Therefore, it can be expected that receiving characteristics become better than that of the MMSE method. However, when the MLD method is used, it is not easy to obtain signal quality SINR for each stream. As is well known, in the MLD method, a symbol group each including a plurality of symbols in a plurality of streams transmitted at the same time is assumed, and the most likely symbol group is specified from among all possible symbol groups so that the transmitted symbol group is estimated. For example, when the number of streams is 2, a symbol group or a symbol pair of s=[s1,s2]T is assumed, in which s1 represents a symbol included in the stream #1, s2 represents a symbol included in the stream #2, and T represents transposition. Assuming that “Q2” is a set of the whole symbol groups, the symbol group SML that is finally detected by the MLD method can be represented as follows.
                              s          ML                =                                            arg              ⁢                                                          ⁢              min                                      s              ∈                              Q                2                                              ⁢                                                r              -              Hs                                                                      (        7        )            SML is a symbol group by which the distance between the actual received signal r and a symbol group s that has received effect of channel variation (represented by channel matrix H) is the minimum, wherein the distance is evaluated by a square of Euclid distance in symbol constellation. Since signal detection is performed for each symbol group from all streams instead of for each stream, it is not so easy compared to the case of the MMSE method to obtain signal quality SINR for each stream. If SINR of each stream is left unknown, it becomes difficult to perform AMC based on the principle shown in FIG. 1. Therefore, it can be considered to estimate SINR for each stream in the following way.
FIG. 3 is a diagram for explaining a method example when adopting the AMC scheme in the MIMO scheme. The functional block in the figure is associated with a link adaptation unit of a conventional communication apparatus. In the example shown in the figure, two streams of the first stream and the second stream are transmitted, and three data modulation schemes of QPSK, 16 QAM and 64 QAM are prepared for the first stream and they are represented as M1, M2, M3 respectively. Also, three data modulation schemes of QPSK, 16 QAM and 64 QAM are prepared for the second stream and they are represented as M1, M2, M3 respectively. In the figure, “for i=1:3 . . . end” indicates performing calculation repeatedly while changing the variable i of data modulation scheme Mi for the first stream. Also, “for j=1:3 . . . end” indicates performing calculation repeatedly while changing the variable j of data modulation scheme Mj for the second stream. K types of channel coding rates are prepared as (R1,R2, . . . ,RK). Also, it is assumed that MCS is selected from the viewpoint of improving the whole throughput achievable in the whole of the first and the second streams.
In the example shown in the figure, first, a symbol error rate is estimated for each stream for a combination of modulation schemes of Mi and Mj by a union bound symbol error probability calculation unit. The symbol error rate SER(m) of a m-th stream is estimated by the following equation (refer to non-patent document 1).
                              SER          ⁡                      (            m            )                          =                              1            K                    ⁢                                    ∑              s                        ⁢                                          ∑                                  c                                                            c                      m                                        ≠                                          s                      m                                                                                  ⁢                              Pr                ⁡                                  (                                      c                    ,                    s                                    )                                                                                        (        8        )                                          with          ⁢                                          ⁢                      Pr            ⁡                          (                              c                ,                s                            )                                      =                  Q          (                                                                      E                  s                                ⁢                                                                                                H                      ⁡                                              (                                                  c                          -                          s                                                )                                                                                                  2                                                                              N                  s                                ⁢                                  σ                  2                                                              )                                    (        9        )            
Meaning of each symbol is as follows.    m: stream index    sm: m-th element of vector s    cm: m-th element of vector c    Pr(c,s): probability by which vector s is erroneously estimated as vector c    K: the number of vector s    Es: total transmission power    Ns: the number of streams    Q( ): Q function
The symbol error rates SER(1)and SER(2) estimated by the union bound symbol error probability calculation unit for each stream are converted into signal qualities (desired signal power to undesired signal power ratio, in the example shown in the figure) SINR(1)and SINR(2) respectively by the AWGN_SNR mapping unit. The AWGN_SNR mapping unit obtains the desired signal power to undesired signal power ratio (SINR), for each stream, that can be achieved with a specific symbol error rate in consideration of additive Gaussian noise. The rate selection unit for the first stream derives a corresponding channel coding rate Rk1 from data modulation scheme Mi and SINR(1). The rate selection unit for the second stream derives a corresponding channel coding rate Rk2 from data modulation scheme Mj and SINR(2).
Correspondence relationship between signal quality and MCS (combination of data modulation scheme and channel coding rate) is predetermined as shown in FIG. 4, for example. The throughput calculation unit calculates a throughput (bps/Hz) based on the combination of the data modulation scheme and the channel coding rate for each stream. Throughputs achievable for each stream are combined, and the combined throughput is shown as “Thr_e” in the figure. When the combined throughput Thr_e is greater than a predetermined threshold Max_Thr, the data modulation scheme and the channel coding rate that have been derived for each stream are set to be a candidate to be used for data transmission.